Chronic Sciatic Neuropathy in Rat Reduces Voluntary Wheel-Running Activity With Concurrent Chronic Mechanical Allodynia.

BACKGROUND: Animal models of peripheral neuropathy produced by a number of manipulations are assessed for the presence of pathologic pain states such as allodynia. Although stimulus-induced behavioral assays are frequently used and important to examine allodynia (ie, sensitivity to light mechanical touch; von Frey fiber test), other measures of behavior that reflect overall function are not only complementary to stimulus-induced responsive measures, but are also critical to gain a complete understanding of the effects of the pain model on quality of life, a clinically relevant aspect of pain on general function. Voluntary wheel-running activity in rodent models of inflammatory and muscle pain is emerging as a reliable index of general function that extends beyond stimulus-induced behavioral assays. Clinically, reports of increased pain intensity occur at night, a period typically characterized with reduced activity during the diurnal cycle. We therefore examined in rats whether alterations in wheel-running activity were more robust during the inactive phase compared with the active phase of their diurnal cycle in a widely used rodent model of chronic peripheral neuropathic pain, the sciatic nerve chronic constriction injury (CCI) model. METHODS: In adult male Sprague Dawley rats, baseline (BL) hindpaw threshold responses to light mechanical touch were assessed using the von Frey test before measuring BL activity levels using freely accessible running wheels (1 hour/day for 7 sequential days) to quantify the distance traveled. Running wheel activity BL values are expressed as total distance traveled (m). The overall experimental design was after BL measures, rats underwent either sham or CCI surgery followed by repeated behavioral reassessment of hindpaw thresholds and wheel-running activity levels for up to 18 days after surgery. Specifically, separate groups of rats were assessed for wheel-running activity levels (1 hour total/trial) during the onset (within first 2 hours) of either the (1) inactive (n = 8/group) or (2) active (n = 8/group) phase of the diurnal cycle. An additional group of CCI-treated rats (n = 8/group) was exposed to a locked running wheel to control for the potential effects of wheel-running exercise on allodynia. The 1-hour running wheel trial period was further examined at discrete 20-minute intervals to identify possible pattern differences in activity during the first, middle, and last portions of the 1-hour trial. The effect of neuropathy on activity levels was assessed by measuring the change from their respective BLs to distance traveled in the running wheels. RESULTS: Although wheel-running distances between groups were not different at BL from rats examined during either the inactive phase of the diurnal cycle or active phase of the diurnal cycle, sciatic nerve CCI reduced running wheel activity levels compared with sham-operated controls during the inactive phase. In addition, compared with sham controls, bilateral low-threshold mechanical allodynia was observed at all time points after surgical induction of neuropathy in rats with free-wheel and locked-wheel access. Allodynia in CCI compared with shams was replicated in rats whose running wheel activity was examined during the active phase of the diurnal cycle. Conversely, no significant reduction in wheel-running activity was observed in CCI-treated rats compared with sham controls at any time point when activity levels were examined during the active diurnal phase. Finally, running wheel activity patterns within the 1-hour trial period during the inactive phase of the diurnal cycle were relatively consistent throughout each 20-minute phase. CONCLUSIONS: Compared with nonneuropathic sham controls, a profound and stable reduction of running wheel activity was observed in CCI rats during the inactive phase of the diurnal cycle. A concurrent robust allodynia persisted in all rats regardless of when wheel-running activity was examined or whether they ran on wheels, suggesting that acute wheel-running activity does not alter chronic low-intensity mechanical allodynia as measured using the von Frey fiber test. Overall, these data support that acute wheel-running exercise with limited repeated exposures does not itself alter allodynia and offers a behavioral assay complementary to stimulus-induced measures of neuropathic pain.

The Efficacy and Safety of the Novel Peripheral Analgesic Isovaline as an Adjuvant to Propofol for General Anesthesia and Conscious Sedation: A Proof-of-Principle Study in Mice.

BACKGROUND: The combination of propofol and an opioid analgesic is widely used for procedural sedation, as well as total IV anesthesia. However, opioids produce respiratory depression, a primary cause of death due to these agents. We recently reported on the antinociceptive actions of isovaline, a small nonbiogenic amino acid that does not readily cross the blood-brain barrier and acts on peripheral γ-aminobutyric acid type B receptors. Here, we explored the possibility that isovaline may be an effective and safe alternative to opioids as an adjunct to propofol for producing anesthesia. METHODS: With approval from our Animal Care Committee, we conducted an in vivo study in adult female CD-1 mice using Dixon's "up-and-down" method for dose assessment. Animals received intraperitoneal saline, propofol, isovaline, fentanyl, or coadministration of propofol with isovaline or fentanyl. We assessed hypnosis by a loss of righting reflex and immobility by an absence of motor response to tail clip application. General anesthesia was defined as the presence of both hypnosis and immobility. We assessed conscious sedation as a decrease in time on a rotarod. The maximal dose without respiratory rates of <4 per minute, apnea, or death was defined as the maximal tolerated dose. RESULTS: Either isovaline or fentanyl coadministered with propofol at its half-maximal effective dose (ED50) for hypnosis produced general anesthesia (isovaline ED50, 96 mg/kg [95% confidence interval {CI}, 88-124 mg/kg]; fentanyl ED50, 0.12 mg/kg [95% CI, 0.08-3.5 mg/kg]). Propofol produced hypnosis (ED50, 124 mg/kg [95% CI, 84-3520 mg/kg]) but did not block responses to tail clip application. Neither isovaline nor fentanyl produced hypnosis at doses which produced immobility (isovaline ED50, 350 mg/kg [95% CI, 286-1120 mg/kg]; fentanyl ED50, 0.35 mg/kg [95% CI, 0.23-0.51 mg/kg]). Isovaline at its analgesic ED50, coadministered with a subhypnotic dose of propofol (40 mg/kg), did not exacerbate propofol-induced deficits in rotarod performance. The median maximal tolerated dose of fentanyl coadministered with the hypnotic ED50 of propofol was 11 mg/kg (95% CI, 8-18 mg/kg). Isovaline at a maximal deliverable (soluble) dose of 5000 mg/kg produced no apparent respiratory depression or other adverse effects. CONCLUSIONS: The novel analgesic, isovaline, coadministered with propofol, produced general anesthesia and conscious sedation in mice. The margin of safety for propofol-isovaline was considerably higher than that for propofol-fentanyl. This study's results show that propofol-based sedation and general anesthesia can be effectively and safely produced by replacing the conventional opioid component with a brain-impermeant peripherally acting γ-aminobutyric acid type B receptor agonist. The results provide proof of the principle of combining a peripheral analgesic with a centrally acting hypnotic to produce general anesthesia. This principle suggests a novel approach to clinical general anesthesia and conscious sedation.

Evaluation of a novel mouse model of intracisternal strychnine-induced trigeminal allodynia.

PURPOSE: Intractable neuropathic dynamic allodynia remains one of the major symptoms of human trigeminal neuropathy and is commonly accepted to be the most excruciatingly painful condition known to humankind. At present, a validated animal model of this disorder is necessary for efficient and effective development of novel drug treatments. Intracisternal strychnine in rats has been shown to result in localized trigeminal dynamic allodynia, thus representing a possible model of trigeminal neuralgia. The purpose of this study was to validate a mouse model of trigeminal glycinergic inhibitory dysfunction using established positive (carbamazepine epoxide) and negative (morphine) controls. METHODS: The actions of conventional first-line treatment (carbamazepine epoxide [CBZe]) and clinically ineffective morphine were tested for trigeminal dynamic mechanical allodynia produced by intracisternal strychnine. In mice under halothane anesthesia, we injected either strychnine (0.3 µg), strychnine with CBZe (4 ng), or artificial cerebrospinal fluid (aCSF) intracisternally (i.c.). In a separate set of experiments, subcutaneous morphine (3 mg·kg(-1) sc) was injected with intracisternal strychnine. Dynamic mechanical allodynia was induced by stroking the fur with polyethylene (PE-10) tubing. The response of each mouse was rated to determine its allodynia score, and scores of each group were compared. In addition, in a separate dichotomous disequilibrium study, pairs of mice were injected with strychnine/saline, strychnine/strychnine-CBZe, or strychnine/strychnine-morphine. A blinded observer recorded which mouse of each pair had the greater global pain behaviour. RESULTS: Strychnine (i.c.) produced higher quantitative allodynia scores in the trigeminal distribution (mean 81.5%; 95% confidence interval [CI] 76.4 to 86.6) vs the aCSF group (mean 11.3%; 95% CI 8.1 to 14.4) (P < 0.0001). Carbamazepine epoxide (i.c.) completely abolished allodynia when co-injected with strychnine (mean 83.2%; 95% CI 78.1 to 88.4) vs strychnine alone (mean 3.2%; 95% CI -0.9 to 7.2) (P < 0.0001). Morphine co-injected with strychnine did not result in reduced allodynia (mean 65.7%; 95% CI 42.0 to 89.4) compared with strychnine alone (mean 87.6%; 95% CI 77.6 to 97.6) (P = 0.16). In a further global allodynia assessment, strychnine (i.c.) produced greater allodynia than both aCSF and strychnine administered with CBZe (P = 0.03). Morphine (ip) administered with strychnine did not result in reduced global allodynia compared with strychnine administered alone (P = 1.0). CONCLUSION: In this study, we have developed and validated a novel murine model of trigeminal dynamic allodynia induced by intracisternal strychnine. The use of mice to study trigeminal allodynia has many benefits, including access to a vast repository of transgenic mouse variants, ease of handling, low cost, and minimal variance of results. The present model may have utility in screening drug treatments for dynamic mechanical allodynia resulting from trigeminal neuropathies.

Effects of local tramadol administration on peripheral glutamate-induced nociceptive behaviour in mice.

PURPOSE: The use of peripheral tramadol to block pain has been advocated. However, since its actions in the periphery have not been elucidated fully, we tested the hypothesis that peripheral tramadol blocks peripheral glutamate-induced nociceptive behaviour in mice. METHODS: First, we compared the duration of paw licking after intraplantar (ipl.) glutamate administration, with and without tramadol, using a randomized blinded controlled design. Next, we established the half maximal effective concentrations (EC(50s)) for local tramadol and reference compound lidocaine in the hot water tail-flick latency test and the glutamate-induced paw allodynia assay. RESULTS: Tramadol reduced glutamate-induced paw licking from 33 +/- 12 sec to 4 +/- 4 sec (mean +/- SD; t test, P < 0.05; n = 6 per group). The tramadol and lidocaine EC(50) nerve conduction blocks in the tail did not differ significantly (84 +/- 24 mM vs 69 +/- 5 mM, respectively). Although tramadol reduced glutamate-induced allodynia (EC(50), 46 +/- 13 mM), lidocaine was more potent (EC(50), 13 +/- 5 mM; Dixon's up-and-down method; P < 0.05). Tramadol was 2.5 times as effective at blocking nerve conduction in the tail compared with allodynia in the paw. CONCLUSIONS: Local tramadol administration blocked nociceptive behaviour in mice induced by peripheral glutamate. Compared with lidocaine, the relative potency of tramadol was lower for blocking glutamate-induced allodynia than for sensory nerve conduction blockade, suggesting the activation of a pronociceptive receptor system in the periphery.